Stability constants of mandelate (2-phenyl-2-hydroxyacetate)
complexes of cobalt(II), nickel(II) and zinc(II) were determined.
potentiometrically using glass and quinhydrone electrodes at
25.0 ± 0.1 aC and ionic strength 2 mole dm" (sodium perchlorate).
The numerical values of stability constants were evaluated from
experimental data by a computer program based on a weighted
least squares method

l. Filipović

T. Matusinović

English

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CROATICA CHEMICA ACTA CCACAA 58 (2) 227-230 (1985)CCA-1573YU ISSN 0011-1643UDC 546.3NoteStability Constants Determination of Mandelate Complexesof Coba1t(II), Nickel(II) and Zinc(II)T. Matusinović* and l. FilipovićLaboratory of Inorganic Chemistry, FacuLty of Technology, University of Zagreb,POB 177, 41001 Zagreb, Croatia, YugoslaviaReceived May lI, 1985Stability constants of mandelate (2-phenyl-2-hydroxyacetate)complexes of cobalt(II), nickel(II) and zinc(II) were determined.potentiometrically using glass and quinhydrone electrodes at25.0 ± 0.1 aC and ionic strength 2 mole dm" (sodium perchlorate).The numerical values of stability constants were evaluated fromexperimental data by a computer program based on a weightedleast squares method.Determination of the composition and stability of metal-ion mandelatecomplexes was performed as a part of systematic investigations of unsubsti-tuted and substituted metal-ion monocarboxylate complexes in our laboratory.The intention was to give some insight into factors governing the stabilityof these complexes-. In order to complete this study the same experimentalconditions (i. e. temperature and ionic strength) have been maintained in thepresent potentiometric investigations of the above mentioned complexes.EXPERIMENTALPotentiometric measurements were performed with potentiograph Metrohm :E436 using a glass electrode (Metrohm EA 109) and a quinhydrone electrode. Volumeaddition reproducibility was ± 0.001 cm? and potential reproducibility was ± 0.2 mY.Solutions were purged with purified nitrogen for 15 min before the measurements.Reagent addition took approx. 30 min; and during this time nitrogen was passedover the surface of the solution which was stirred magnetically.All determinations were carried out at 25.0 ± 0.1 aC and at ionic strength 2mole dm= (sodium perchlorate).Each system was studied at two metal-ion concentrations, both of these at twoacid-to-salt ratios 1 : 1 and 1: 2. Concentrations of Co(II) and Ni(rI) were 20 and10 mmole dm? and of Zn(II) 10 and 5 mmole dm'". The total ligand concentrationwas up to 100 mmole dm", because at higher Iigand concentration precipitationoccurred.Cobalt(II) and nickel(II) perchlorates were prepared from perchloric acid andthe corresponding carbonates, while zinc(II) perchlorate was prepared from thecorresponding oxide and perchloric acid. All salts were recrystallized three times.Metal-ion concentrations in the stock solutions were determined by complexometricpotentiometric titration with a mercury indicator electrode". Concentrations of freeacid in metal perchlorates were also determined and taken into account when pre-* Present address: SOUR CHROMOS, Zagreb c. Đure Đakovića 43228 T. MATUSINOVIC AND I. FILIPOVICpar ing the buffer solution. Analytical grade dl-mandelic acid (Merck), sodiumperchlorate and sodium hydroxide were prepared by dilution of correspondingstock solutions.RESULTS AND DISCUSSIONExperimental data were treated with precision of 10.5 significant digitson the PDP-8 computer programmed to calculate average number, n, ofJigand bound to the central metal-ion, the free-ligand concentrations, [LJ,and the values of the functionNFo «(L]) = 1 + ~ (Ji (L];i=1A weighted least-squares procedure was then used to evaluate the stabilityconstants". Logarithmic values of stability constants are listed in Table 1.TABLE IStability Constants of MandeLate CompLexes t = 25aC, I = 2 moL dm-3 (NaCLQ.j)Metal ion MethodCo'+ Pot. (GE)*Pot. (QE)**NP+ Pot. (GE)Pot. (QE)Zn2+ Pot. (GE)Pot. (QE)1.19 ± 0.011.26 ± 0.011.31 ± 0.011.35 ± 0.011.36 ± 0.011.40 ± 0.012.07 ± 0.011.95 ± 0.012.48 ± 0.012.40 ± 0.012.58 ± 0.012.45 ± 0.01* GE 1 Glass electrode**QE Quinhydrone electrodepKA = 3.33 (our value)A good agreement between the formation curves? obtained for differentmetal-ion concentrations and acid-to-salt ratios eliminates the existence ofpolynuclear complexes, and rules out the existence of hydroxy complexes.The behaviour of glass and quinhydrone electrodes during potentiometriccomplex formation measurements was investigated earlier", It was shownthat a difference in log K, of approximately 0.05 was obtained and that mono-ligand, stability constants obtained by the glass electrode were always lowerthan those obtained by the quinhydrone electrode. It was demonstrated thatthis difference can be attributed to deviation from the Nernst equation. Thepotential measured with the quinhydrone electro de was higher while : thepotential measured with the glass electrode was lower than the value calcu-lated from the Nernst equation.The standard error values in Table I of 0.01 logarithmic units expressinner consistency of the measured data. An attempt was made to estimatea realistic error in stability constants determination. All potentials were mea-sured to ±0.2 mV so the set of data was changed by 0.4 mV and with thesenew values of potentials a new set of stability constants was calculated. Thenan error was assumed in ligand concentration of 10 per cent and in metal ionconcentration of 5 per cent. All these changes did not give a greater differencein log 1 of 0.03, which proves that the design of the experimental method isMANDELA TE COMPLEXES 229very good because the stability constants are obtained from the difference inpotential measured for the solution of ligand in the presence and absence ofmetal ioncomplex formation. Most errors are canceled through this difference.Therefore, it can be estimated that all the results presented in Table I arerealistically confident to ±0.03 logarthmic units.On the basis of the stability constants of monoligand complexes, thestability of the investigated systems follows the order Co Ni Zn, which isthe same order as stated in this laboratory for all other unsubstituted andsubstituted monocarboxylate complexes-,Lower stability of mandelate complexes of investigated bivalent metal--ions in comparison with the stability of the corresponding glycolate complexesis in accordance with the lower basicity of mandelate ligand (pKA 3.33) dueto the inductive effect of C6Hs-group. The stability of the metal-ion complexis lower in the same proportion as that of the proton complex i. e. the contri-bution to the stability due to chelate coordination of OH-group is approxi-mately one logarithmic unit, which is the same as in glycolate complexes-where the chelate coordination was proved by IR in vestigations", The samewas determined for the investigated mandelate complexes by IR studies ofP. V. Khadikar et all." This points to the conclusion that the inductive effectof C6Hs-group in the investigated ligand is independent of the central ion.The values of monoligand stability constants obtained in our investi-gations are in good agreement with Folkesson and Larrsons data",It should be mentioned, however, that these authors investigated thesystem within a broader ligand concentration range (up to 0.16 mol dm")and that they also encountered the precipitation of the complexes from thesolution, which prevented the investigation at a higher ligand concentration.In our experience, by using the analytical grade dl-mandelic acid (Merck)it was not possible to prepare the investigated complexes in solution withmore than 0.10 mol dm'" of ligand concentration, because at higher concen-trations precipitation occurred immediately. In our experiments and in thoseof Folkesson and Larsson approximately the same metal ion concentrationswere used. This is probably the reason why monoligand stability constantsare in good agreement between the results presented in this paper and thosepresented in Folkesson's and Larsson's paper. Biligand stability constants arein fair agreement and we were not able to detect the exsistence of the thirdcomplex in solution within our experimental conditions.Acknowledgement - The authors are indebted to the Scientific Research Concilof Croatia for financial support.REFERENCES1. 1. F i 1i p o v i ć, Kem. ind. 30 (1981) 369.2. B. G rab ari ć, B. Mayer, 1. P i 1j a c, and 1. F i 1i p o v i ć, Electrochim.Acta 20 (1975) 799.3. G. Seh war zen b a c h, Die Komplexometrische Titration, F. Enke, Stuttgart,1960.4. B. F o 1k e s s o n and R. L ars s o n, .4.cta Chem. Scand. 22 (1968) 1953.5. R. L ars s o n, Acta Chem. Scand. 19 (1965) 783.6. P. V. K had ika r, R. L. Ame ria, M. G. K e k r e, and S. D. C h a uha n,J. Inorg. Nucl. Chem. 35 (1973) 4301.7. 1. Kr uha k and 1. F i 1i p o v ić, Croat. Chem. Acta 52 (1979) 207.I230 To MATUSINOVIC AND 1. FILIPOVIĆSAŽETAKIstraživanje stabilnosti mandelato-kompleksa kobalta(II), nikla(II)To Matusinović i lo FilipovićKonstante stabilnosti mandela to-kompleksa kobalta(II), nikla(II), i cinka (II)određene su potenciometrijskom titracijom u tamponskim otopinama stalne ionskejakosti 2 mol dm? (NaCI04)ocinka(II)

Stability Constants Determination of Mandelate Complexes of Cobalt(II), Nickel(II) and Zinc(II)

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